ath9k: fix aggregation related interoperability issues

Some clients seems to keep track of their reorder window even after an
aggregation session has been disabled. This causes issues if there are
still retried but not completed frames pending for the TID.
To ensure that rx does not stall in such situations, set sendbar to 1
for any frame purged from the TID queue on teardown.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>